There are many heating and cooling systems on the market and many of these are reliant on fossil fuels. With the ever increasing demand for more environmentally friendly systems various alternative systems based on sunlight or water have been proposed such as for example photovoltaics, solar thermal electricity generators, hydroelectricity, wave power and bio-fuels.
An issue common to all solar-driven renewable energy conversion devices, some hydro-driven devices, and wind turbines is that they cannot operate “on demand”, as the sun does not always shine, the seas are not always high and the wind does not always blow. This means that at some times these so-called intermittent renewable sources will generate electricity which cannot be easily integrated into their corresponding local electricity grids, and as such there have been a number of storage solutions proposed.
The thermal energy storage system, proposed in WO 2009/138771 converts surplus electrical energy from intermittent renewable sources into heat or cool when available, store the so-converted heat or cool in a thermal store, and then make it available as useful heat or cool on demand using phase change materials (PCMs) to effect the energy conversion via their inherent solid-liquid phase changing properties.
For practical application in domestic situations phase change materials capable of supplying warm, or even hot, water that is just above the comfort level temperature of the individual requiring the heated water are needed. In addition, phase change materials suitable for such practical utility should achieve rates of heat transfer into and out of, their phase changes, which are commensurate with domestic use, as well as deliver acceptable levels of thermodynamic stability (efficiency).
Sodium acetate trihydrate, (SAT), has a solid-liquid phase change within the desired temperature range for domestic utility. However, the practical applications of SAT as a phase change material (PCM) are limited by the unique and incongruous manner in which it melts, going from solid SAT, to a mixture of liquid SAT and a solid (sodium acetate/SA) at a set temperature, 58° C. This inability to provide a fully liquid solution at 58° C. is an issue which is reflected in the thermodynamic stability of aqueous solutions upon initial heating, as well as the thermodynamic stability of, re-formed, aqueous solutions provided following re-heating after cooling in accordance with the normal heating/cooling cycles in phase change systems.
This solid-formation on melting is a problem for use as SAT as a PCM because this solid SA, once formed, would ordinarily be retained throughout the lifetime of the PCM in a phase change system.
Previous attempts to overcome this issue via the utility of three dimensional cross-linked polymers, such as cellulose based polymers and super absorbent polymers, to act as solid supports have been unsuccessful because, independent of any apparent initial reduction of solid formation, the underlying problem of solid sodium acetate formation is unresolved, and over time solid sodium acetate will still precipitate out of solution, and collect at the base of the PCM storage vessel irreversibly. For utility as a PCM, no solution which has a finite lifetime is acceptable.
It is an object of at least one aspect of the present invention to obviate or mitigate at least one or more of the aforementioned problems in relation to the utility of sodium acetate trihydrate as a potential PCM for use in aqueous phase change systems.
It is an object of at least one aspect of the present invention to provide improved phase change materials containing sodium acetate trihydrate having desirable homogeneity; resistance to SA formation, which are suitable for use in phase change systems.
It is an object of at least one aspect of the present invention to provide improved phase change materials containing sodium acetate trihydrate which can be heated, cooled and re-heated in repeated cycles with retention of thermodynamic stability.
The Applicant has developed novel and inventive aqueous compositions for use as phase change materials comprising: sodium acetate trihydrate; at least one alkali soluble polymer for inhibition of sodium acetate anhydrous crystal formation in sodium acetate trihydrate containing phase change materials; and at least one sodium acetate trihydrate nucleation promoter. The Applicant has also developed a process for the preparation of said improved phase change materials.